1. Field of the Invention
Aspects of the present invention relate to a fuel cell system, and more particularly, to a fuel cell system that facilitates the supply of a liquid fuel to a reformer using the pressure from a gaseous fuel tank.
2. Description of the Related Art
A fuel cell is an electricity generation system that transforms chemical energy directly into electrical energy through a chemical reaction between oxygen and hydrogen contained in hydrocarbon groups of such molecules as methanol, ethanol, or natural gas.
A fuel cell system includes a fuel cell stack and a fuel processor (FP) as main components, and a fuel tank and a fuel pump as auxiliary components. The fuel cell stack has a structure in which a few to a few tens of unit cells are stacked. Each unit cell comprises a membrane electrode assembly (MEA) and a separator on both sides of the MEA.
A fuel pump supplies a fuel from the fuel tank to a fuel processor, in which a reformer produces hydrogen by reforming the fuel and supplies the hydrogen to the fuel cell stack. The hydrogen in the fuel cell stack reacts electrochemically with oxygen to generate electrical energy.
The fuel processor reforms hydrocarbon groups and chains using a catalyst. If the molecule contains a sulfur compound, the catalyst can be easily poisoned by the sulfur compound. Therefore, the sulfur compound must be removed from the molecule before the hydrocarbon group or chain may be processed in the reformer. Accordingly, the hydrocarbon is processed in a desulfurizing process before the hydrocarbon is fed to the reformer.
The hydrocarbon produces hydrogen while it is being reformed, and also produces carbon dioxide and a small amount of carbon monoxide. However, the carbon monoxide acts as a catalyst poison to the catalytic layer of the electrodes in the MEA. Therefore, the reformed fuel must be processed in a shift unit in which the concentration of carbon monoxide is decreased before the fuel is supplied to the fuel cell stack. At this time, the concentration of the carbon monoxide may be decreased to less than 5000 ppm.
FIG. 1 is a configuration of a conventional fuel cell system.
Referring to FIG. 1, in a fuel cell system that uses a gaseous fuel, the gaseous fuel may be simultaneously supplied to a reformer 40 and a reformer burner 30 from a gaseous fuel tank 10. Desulfurizer 14 removes sulfur compounds from the gaseous fuel so as to preserve the catalytic materials within the reformer 40. Gaseous fuel, such as liquefied petroleum gas or LPG, that has passed through the desulfurizer 14 must have a sulfur concentration of less than 1 ppm.
The reformer burner 30 heats the reformer 40 to maintain the reformer 40 at a temperature of approximately 750° C. The combustion gas from the reformer burner 30 is exhausted to the atmosphere after having transferred heat to the liquid fuel and water flowing through a first heat exchanger 71.
A liquid pump 22 supplies water to the reformer 40 from a water tank 20. The water supplied to the reformer 40 by the liquid pump 22 is preheated by passing through first and second heat exchangers 71 and 72.
The reformer 40 generates hydrogen, carbon dioxide, and carbon monoxide from the hydrocarbon groups and water. A shift reactor 60 decreases the concentration of carbon monoxide in the fuel produced at the reformer 40 to a predetermined level, such as 5000 ppm or less, before the hydrogen-rich gas is supplied to the fuel cell stack 50.
In the fuel cell arrangement of FIG. 1, a gaseous fuel containing sulfur is used as a fuel to supply to the reformer 40. Therefore, the desulfurizer 14 is necessary to remove such sulfur from the gaseous fuel before entering into the reformer 40. Also, this arrangement requires that the liquid pump 22 supplies water to the reformer 40.
A liquid fuel that does not include sulfur can be used instead of the gaseous fuel including sulfur. However, in this case, an element for supplying the liquid fuel to the reformer burner 30 is required.